Scientia Iranica
Volume:22 Issue: 1, 2015

A di erent numerical approach for the two dimensional nonlinear Fredholm integral equations of the second kind with the continuous kernel is considered. The main idea is to convert the integral equation into an optimization problem. Then by using an embedding method, the class of admissible trajectories is replaced by a class of positive Borel measures. The optimization problem in measure space is then approximated by a nite dimensional Linear Programming (LP) problem. Some examples demonstrate the e ectiveness of the method.

In the area of bluff body aerodynamics, controlling the vortex shedding is of great importance to prevent damages caused by the vortex-induced forces. Accordingly, a short and thin splitter plate has been utilized at different positions downstream to interfere with the vortex wake of a confined square cylinder in the framework of an optimal passive control study. Vorticity-stream function formulation and finite difference method have been used to numerically simulate the two dimensional laminar flow. For preventing the flow from becoming three dimensional, the Reynolds number is kept below 250. Validation of the code and mesh dependency analysis are offered. Based on the flow patterns acquired, the optimal position of the splitter plate behind the cylinder can prevent the detachment of the pair of vortices from the cylinder and will keep the streamlines symmetric around the zero streamline. Consequently, vortex shedding does not occur and the vortex-induced forces are eliminated entirely. Interestingly we showed that the vortex can be suppressed even when the splitter plate is asymmetrically arranged behind the cylinder, especially in the case of positioning the body close to the surrounding walls.

Several kinetic descriptions have already been utilized for the simulation of rarefied gas mixture flows. Although such developments are important, Navier-Stokes computation can find extended use in engineering applications. Recently, the authors have derived new velocity-slip and temperature-jump boundary conditions for slip flows of gas mixtures. Appealing to these new boundary conditions, Navier-Stokes computation of rarefied gas mixture problems has become feasible.In the present contribution, the proposed conditions in conjunction with the Navier-Stokes equations and an equation for the conservation of species are solved for some binary gas mixture problems in the slip flow regime. Applications include low pressure flow in a converging-diverging nozzle, wall-cooling of a nozzle under rarefied condition, and parallel mixing in a microchannel. Simulation results are presented in terms of the distributions of overall Knudsen number, Mach number, pressure, temperature, and concentration.

A finite element formulation based on two-variable refined plate theory has been developed in this paper and has been implemented for bending analysis of isotropic and orthotropic plates. The two-variable refined plate theory can be used for thin and thick plates and predicts parabolic variation of transverse shear stresses across the plate thickness. In this theory the zero traction condition on the plate surfaces is satisfied without using shear correction factor. The governing equations have been derived using the principle of minimum potential energy. After constructing weak form equations، a new 4-node rectangular plate element with six degrees of freedom at each node has been used for discretization of the domain. The finite element code is written in MATLAB and some benchmark problems have been solved. Comparison of results with exact solution and other common plate theories shows the accuracy and efficiency of presented finite element formulation.

Natural convection flow is considered in an annular enclosure that is filled with a fluid-saturated porous medium. The top and bottom horizontal walls are insulated while outer vertical wall is cooled with a constant temperature and a linear temperature gradient applied to inner vertical wall.Dimensionless equations are solved numerically using Finite Volume Method (FVM) on a collocated non-uniform, orthogonal grid. Darcy–Forchheimer model is used to simulate the momentum transfer in the porous medium. The effect of Rayleigh, Darcy, Prandtl, solid-fluid Nusselt, heat generation parameters and effective conductivity ratio of phases on the streamlines and isotherms are presented, as well as on the rate of heat transfer from the inner and outer vertical wall of the enclosure. It is concluded that at low values of effective conductivity ratio of phases, only the heat generation parameter has significant effect on the temperature field of the solid phase and the heat transfer from inner and outer vertical walls. An analytical solution is obtained for this case and compared with numerical solution. The resultsshow good agreements together.

Cold working a hole decreases tendency of fatigue crack near the hole to initiate or growth. It is due to creation some compressive tangential residual stresses around the hole. In this paper strains recorded by strain gages mounted around the hole during cold working process has been introduced as a feature for residual stress field. Finite element modeling has been used for verifying the experimental results, which showed a good agreement. Also fatigue test has been performed on specimens with and without cold working which showed increasing of fatigue life.

The goal of this paper is to introduce and optimize a power split continuously variable transmission (PS-CVT) including several fixed ratio (FR) mechanisms. In this power train, FR mechanisms are embedded in all possible places. At first, the governing dynamics of the proposed power train is developed. A control algorithm on the base of minimizing the vehicle fuel consumption (FC) is introduced. Two vehicles in different classes are considered and the proposed power train is used in them. Afterwards, two stages optimization with the aim of minimizing the vehicles’ FC is carried out. It is found that employing properly designed FR mechanisms improves the power train and decreases the vehicles’ FC. Also, it is revealed that there is a specific power train for each of the vehicles that gives a remarkable fuel economy in each of the considered driving cycles.

Aquifers are underground porous formations containing water. Confined aquifers are formations surrounded by two impermeable layers. These aquifers are suitable for seasonal thermal energy storage. The objective of this research is a parametric study of the pressure distribution in an aquifer to be employed for thermal energy storage for air-conditioning of a building complex. In design of an aquifer thermal energy storage (ATES), a realistic model is needed to predict the aquifer’s behavior. Here, the effects of operating parameters on pressure distribution are investigated through a three-dimensional finite difference model. In an ATES, heat transfer occurs through both convection and conduction. The convective heat transfer in ATES occurs because of pressure gradient. Therefore, knowledge of the effects of various parameters on pressure distribution is necessary. These parameters are: groundwater natural flow, porosity and permeability of the aquifer, injection and withdrawal rates from wells, number and the arrangements (being linear, triangular or rectangular) of injection and withdrawal wells. It has been found that the variation of the pressure drop inside the aquifer with increasing permeability is very considerable in comparison with other parameters. Moreover, a validation is performed by using the Fluent software to verify the accuracy of the developed method.

This paper proposes and investigates a combined ejector-cooling and power (CECP) system, using R123 as a working fluid to utilize the solar energy over a low temperature range. Evacuated tube solar collectors are used to collect the solar radiation for their low costs. A thermal storage system and an auxiliary boiler are used to provide continuous cooling and power output when solar radiation is not sufficientMathematical models are employed to simulate the system under steady-state conditions. The results obtained reveal that solar collector and auxiliary boiler are the main exergy destruction sources. Parametric analysis is conducted to examine the effects of some key thermodynamic parameters on the system performance. The results indicate that under the actual constraints, increasing turbine inlet pressure elevates system efficiency while increasing turbine inlet temperature and turbine back pressure decreases that.The system is also optimized with the energy and exergy efficiencies as objective functions by means of genetic algorithm under the given conditions.

Bone drilling is a common surgical procedure in orthopaedic, neuro, and dental surgeries for internal fixation. Estimation and control of bone drilling force and torque are critical to prevent drill breakthrough, excessive heat generation, and unnecessary mechanical damage to the bone. This paper illustrates the experimental measurements and comparison of drilling thrust force and torque in conventional drilling (CD) and ultrasonically-assisted drilling (UAD) in two directions, i.e., along the longitudinal axis of the bone and normal to it (radial direction). The objective was to find the effect of drill size and ultrasonic vibrations superimposed on the drill’s movement on the thrust force and torque. The effect of drill speed on force was investigated in the described directions, followed by a series of experiments to explore the influence of drill size and penetration direction on the level of force and torque. The drilling force and torque were found to be strongly dependent on the drilling direction. Experimental results revealed that drilling in direction, lower drilling force and torque were found in UAD compared to CD.

This paper illustrates the controllability condition for linear stochastic fractional systems. The definition of fractional stochastic controllability is given. The - controllability matrix has been presented to derive the required theorems for necessary and sufficient conditions of complete and approximate fractional stochastic controllability. The equivalency of fractional stochastic controllability to fractional controllability is also investigated. An example has been given to examine the effectiveness of the theory.

In this paper, three adaptive critic-based neuro-fuzzy controllers are presented for improving attitude and position control of ships. The controllers include vessel position and heading errors and their derivatives as inputs. Three critic based reinforcement learning methods evaluate the situations of the ship in terms of satisfactory achievements of the ontrol goals. The critic agent output, namely the reinforcement signal, is a measure of the controlled system stress. The controller modi es its characteristics in a way that the critic stress is decreased. The proposed controller has a simple structure and shows satisfactory transient responses and robustness to model uncertainty.

Age hardening behavior and microstructure of superalloy AEREX 350TM in commercially cold-drawn condition were studied. It was found that aging the cold drawn alloy significantly facilitated the formation of major phases of gamma prime and eta, in AEREX 350TM. Interestingly, the minimum temperature of formation of gamma prime and eta phases in the cold drawn alloy was lower by about 30°C and 5°C, respectively, to that previously reported for the solution treated alloy. Cold working, however, did not considerably influence the maximum temperature of stability of these phases. In addition, Widmanstätten morphology of eta phase significantly improved in the cold drawn samples compared to that seen in the solution treated alloy. Diffusion through dislocation network could have facilitated growth of major phases whereas its influence on dissolution temperature of these phases was negligible. In addition, it was suggested that cold working might increase the density of hcp nuclei in the fcc matrix. This may justify the observed enhancement of the Widmanstätten morphology of eta phase in these samples following the aging process.